I'm thinking your PTO is really a Power Transfer unit that disengages the rear axle when the compressor is engaged, fire trucks with mid-mount pumps use this type on set up. The lever in the steering column would be a manual brake line lock, you stand on the brake pedal and the pull the lever down to lock and hold the pressure in the brake lines. This is needed since the parking brake on the back of the trans can't be used while the trans is powering the compressor. The gauge shows the pressure in the system so you can keep an eye on it, they would leak down requiring a reset over time.
Make sure you chock your truck before you start playing with this unit to keep it from rolling away!
Maybe this thread should be moved to the Big Bolt or driveline section...
Mike B
